From DE 10 2009 049 280 A1 an active sound absorber is known, which has a housing and a connecting pipe for the acoustic and fluidic connecting of the housing with the exhaust system. A loudspeaker is arranged in the housing, which comprises an active membrane and an actuator for vibration stimulation of the membrane. In the housing, the membrane separates a front volume, connected fluidically with the connecting pipe, from a back volume.
Such active sound absorbers are used, by feeding in a calculated sound, in particular counter-sound or anti-sound, to influence an exhaust noise of the exhaust system in a desired manner, preferably to damp it. For this, the front volume is in fluidic connection with the exhaust system via the connecting pipe. The front volume typically has no direct connection to the atmosphere outside the exhaust system, i.e. to the environment of the exhaust system. The back volume is delimited by the active membrane and the housing of the sound absorber, so that the loudspeaker operates on the rear side on a closed volume and on the front side on the exhaust system.
Due to the type of construction, the membrane of such a loudspeaker with an electrodynamic actuator is sensitive with respect to different static or respectively quasi-static pressures in front and behind the membrane. Depending on the area of the membrane and the rigidity of a membrane suspension, the membrane of the loudspeaker is deflected from the central position by a differential pressure, which reduces the capability of the loudspeaker to generate dynamic alternating pressures in front of and behind the membrane through its electrodynamic drive (actuator). If this deflection from the central position continues furthermore over a longer period of time and additionally under thermal stress of the loudspeaker, the membrane can remain permanently deflected owing to the creep behaviour of individual components of the loudspeaker, in particular of the membrane suspension, also without a pressure difference existing furthermore between front volume and back volume and acting on the membrane.
The differential pressures occurring in this connection between front volume and back volume can be roughly differentiated from one another as follows. On the one hand, a static pressure difference occurs by an alteration of the outer air pressure in the atmosphere or respectively environment of the exhaust system as a result of the weather, e.g. on a change from a low pressure area to a high pressure area or as a result of a change to the height above sea level, e.g. when driving uphill. These static pressure changes occur relatively slowly, for example with a time constant or period duration of more than 10 sec., i.e. with a frequency of less than 0.1 Hz. Furthermore, a quasi-static pressure difference occurs by altering the flow conditions in the exhaust system, in particular by the Bernoulli effect at the junction between the connecting pipe and the exhaust system. The flow conditions in the exhaust system change as a function of the respective operating state of the internal combustion engine, for example on a change from idle mode to higher loads or full load, which is involved with higher mass flows and exhaust gas temperatures. These quasi-static pressure changes occur for example with a time constant or period duration of between 0.1 sec. and 10 sec., i.e. with a frequency between 0.1 Hz and 10 Hz. Finally, dynamic pressure differences can also occur, namely the alternating pressures generated conventionally by the loudspeaker, i.e. the acoustic signals for influencing the acoustic emission of the exhaust system. These dynamic pressure fluctuations typically have a period duration or respectively time constant of less than 0.1 sec., i.e. frequencies greater than 10 Hz.
In order to ensure the proper function of the electrodynamic loudspeaker, i.e. the assembly of active membrane and associated electrodynamic actuator, therefore all differential pressures with a period duration greater than 0.1 sec., i.e. the static and quasi-static pressure fluctuations, must be equalized. At the same time, it must be ensured that in the relevant frequency range from 10 Hz the electrodynamically generated alternating pressures are not substantially reduced or even acoustically short-circuited.
A compensation or equalization of the static pressure differences, i.e. of the slow fluctuations of the atmospheric air pressure with respect to the closed back volume can be achieved in that at least one relatively small pressure equalization opening is provided, which fluidically connects the back volume with the environment of the sound absorber. Under certain circumstances here a slight permeability of the housing can already be sufficient in order to equalize the static pressure differences.
According to DE 10 2009 049 280 A1 mentioned in the introduction, an equalization of the quasi-static pressure fluctuations can be enabled by at least one pressure equalization opening, which fluidically connects the back volume with the front volume. Such a pressure equalization opening is dimensioned here so as to be comparatively small, in order to avoid an acoustic short-circuit between front volume and back volume.
Such pressure equalization openings between front volume and back volume are gas-permeable and open to diffusion, whereby in particular exhaust gas, which arrives into the front volume via the connecting pipe from the exhaust gas system, can also enter into the back volume. Here, at the same time, a temperature gradient occurs, because the exhaust gas in the exhaust system is generally exposed to higher temperatures than in the back volume. The problem arises here that humidity linked in the exhaust gas, i.e. vapour, condenses in the cooler back volume. Depending on the exhaust gas composition, the condensate occurring here is comparatively aggressive, in particular the condensate can comprise sulphuric acid. In the long run, the aggressive condensate can damage the electrodynamic actuator and connecting cable. Measures for improving the condensate resistance at the loudspeaker and the insulation of the cable and the connection between the cables and the actuator are comparatively laborious and increase the production costs. If one avoids these cost-intensive measures for improving the condensate resistance, the active sound absorber can only be positioned on the exhaust gas system in the region of a tailpipe, wherein by structural measures at the respective tailpipe, provision can be made that the quasi-static pressure difference between front volume and back volume, brought about by the flow speed, is then as small as possible. Consequently, the pressure equalization opening between front volume and back volume can be dispensed with. However, this significantly restricts the configuration of the active sound damping and impedes or respectively prevents the use of an active sound absorber at a region distant from the tailpipe upstream in the direction of the engine, although the acoustic effectiveness of the active sound absorber is possibly better there.